The present invention relates to a memory device, and, more particularly, to a technique applicable to a semiconductor memory device using a resistance change element.
One non-volatile memory is a resistance change memory (ReRAM) using a difference in a resistance value between a low-resistance state (On state) and a high-resistance state (Off state) in the resistance change element.
For example, the publication of Japanese Patent No. 4838399 describes a method for improving the stability, the reliability, and the memory retention characteristic of a writing operation in the ReRAM. The resistance change element has some characteristics. Specifically, the first resistance state is changed to the second resistance state, upon application of a first voltage pulse, and the second resistance state is changed to the first resistance state, upon application of a second voltage pulse having a polarity different from that of the first voltage. When to change from the first resistance state to the second resistance state, according to the described method, applied to the resistance change element are at least the first voltage pulse, a third voltage pulse whose voltage absolute value is smaller than that of the second voltage and having the same polarity as that of the second voltage, and the first voltage pulse, sequentially in this order.
The publication of Japanese Patent No. 5250726 describes a method including a weak writing step and subsequently a normal writing step, as steps for changing a resistance change element from the first resistance state to the second resistance state. In the weak writing step, a third voltage pulse having the same polarity as that of the first voltage and having a different absolute value is applied to the resistance change element, thereby causing it to be in the second resistance state. After this, a fourth voltage pulse having the same polarity as that of the second voltage and having the absolute value lower than that of the second voltage and the third voltage is applied, thereby causing it to be in an intermediate resistance state having a resistance value between the first resistance state and the second resistance state. In the normal writing step, the first voltage pulse is applied at least once to the resistance change element, thereby transiting from the intermediate resistance state to the second resistance state.